Implementations based on conventional techniques use small solid-state LED and laser components of sufficiently high power output in the visible and near-IR wavelength range. Also used are wide-band emitting halogen-cycle incandescent lamps and so-called "globar" sources.
The solid-state LED and laser components are extremely useful sources of radiation in absorption measurements by virtue of their small size and low power consumption. Their disadvantage is that their output wavelength does not necessarily coincide with the absorption wavelength band of the gas or liquid to be measured. Furthermore, the longest wavelength available from them rarely exceeds 1.5 .mu.m. Incandescent lamps and globar sources are radiation sources characterized by large size and high power consumption, and their emission spectrum closest approximates the wide-spectrum radiation of a blackbody. The desired wavelengths from such a spectrum must generally be selected by means of filters, which make the construction mechanically complicated, particularly when the filters must be movable for modulation purposes.
U.S. Pat. No. 3,725,701 shows a gas analyzer in which the gas to be analyzed is placed in a container and illuminated with IR radiation emitted by a blackbody. The fluorescence radiation produced in the gas is then employed for the detection of the gas. A disadvantage of this implementation is that it needs a hot blackbody of high energy consumption for use as the emitter of the primary radiation employed for exiting the gas under study. The gas under study behaves like a narrow-band filter, which separates the desired wavelengths from the blackbody radiation. Consequently, the radiation intensity from the gas under study cannot possibly be higher than that of the exciting radiation from the blackbody at the narrow wavelength band in question. Therefore, the available intensity from the gas under study necessarily remains very low, down to a fraction of the total intensity of the exciting radiation emitted by the blackbody.
The EP patent application 231 639 discloses a light source implementation in which a gas is excited with the help of an electric discharge. A disadvantage of such an implementation is that the gas contained in the light source is ionized in the electric discharge. Ionization in turn, however, implies that there is a high gas temperature and Doppler broadening of the spectral lines on one hand, and, on the other hand, that the gas is in a reactive state and undergoes a gradual change in its composition due to chemical reactions. Therefore, the lifespan of light source is limited to a few thousand hours, even if the light source is provided with a gas reservoir, which in turn makes the light source bulky.